1. Field of the Invention
The present invention relates to a beam irradiation apparatus which is suitably used in an inter-vehicle distance detector, a distance detector, and the like.
2. Description of the Related Art
Recently, a detection apparatus in which a target region is irradiated with a laser beam to detect an obstacle within the target region is mounted on a passenger automobile. In such a detection apparatus, the laser beam is scanned in a horizontal direction and a vertical direction within the target region, and a distance between the automobile and the obstacle is detected from an acceptance state of the reflected light.
In order to scan the laser beam, a so-called beam irradiation apparatus is arranged in the detection apparatus. For example, a lens actuator disclosed in Japanese Patent Publication Laid-Open No. 11-83988 can be used to scan the laser beam. In the actuator, a scanning lens converts the laser beam (diffuse light) emitted from a light source into a parallel beam or a light beam slightly diffused rather than the parallel beam. The scanning lens is two-dimensionally driven in a direction perpendicular to an optical axis of the laser beam according to the drive of the actuator, which allows the laser beam to be scanned within the target region.
However, in the conventional technique, there is generated a problem a swing width of the laser beam is restricted by a drive amount of the actuator. Because the drive amount of the actuator has a predetermined restriction, in order to displace the scanning lens beyond the restriction, it is necessary to enlarge the actuator, or it is necessary to enhance drive force of an electromagnetic circuit including a magnet and a coil. However, the beam irradiation apparatus is therefore enlarged to generate a further problem that power consumption is increased.
In the conventional beam irradiation apparatus, generally the target region is divided into matrixes in the horizontal direction and the vertical direction, an outgoing level of the laser beam is enhanced in a pulsating manner at timing a scanning position of the laser beam reaches each grid position, and the grid position is irradiated with the laser beam. The detection whether or not the obstacle exists at the grid position is performed based on whether or not the light reflected from the target region is detected at each grid position, and a distance to the obstacle is detected based on a time difference between outgoing timing and acceptance timing of the laser beam.
In this case, resolution of the grid has an influence on accuracy of the obstacle detection. That is, as the resolution of the grid is enhanced, the accuracy of the obstacle detection is enhanced in the target region. At the same time, when the resolution of the grid is enhanced, an outgoing frequency of the laser beam is increased, which complicates scanning control. On the other hand, in the passenger automobile it is necessary that the detection accuracy in the horizontal direction be enhanced rather than the detection accuracy in the vertical direction. Accordingly, it is desirable that the resolution of the grid be enhanced in the horizontal direction while lowered in the vertical direction.